1. Field of the Invention
The present invention relates to unity gain buffer amplifiers for analog signals, and in particular, to analog signal amplifiers with true unity gain, full rail-to-rail dynamic signal range and symmetrical load current sourcing and sinking capabilities.
2. Description of the Related Art
It is well known that unity gain buffer amplifiers provide an essential building block for analog systems. Ideally, such an amplifier should have infinite frequency response, be capable of driving infinite loads while drawing only minimum power, and have no DC offset. The reality, however, is that there is always a number of compromises which must be made with respect to power dissipation, dynamic signal range, signal frequency response and transient response.
Conventional unity gain buffer amplifiers are typically variations on one of two general configurations. Referring to FIG. 1, one configuration involves an operational amplifier connected in a unity gain topology. This configuration is very common and has numerous desirable features. However, the frequency response is generally quite limited due to the high impedance nodes within the amplifier which form low frequency poles. Further, the group delay within the amplifier causes transient intermodulation distortion which makes it undesirable for high frequency applications.
Referring to FIG. 2, another common configuration involves a source follower. This configuration has good frequency response but has a gain of less than unity (typically 0.8-0.9 in metal oxide semiconductor (MOS) technology), limited dynamic signal range and unsymmetrical load current sourcing and sinking capabilities. Further, there is a voltage shift from the input (gate terminal) to the output (source terminal) of one threshold voltage drop Vt (approximately 1 volt). Such a voltage shift may be acceptable in 5-volt designs but is generally an unacceptable limitation in lower voltage designs, such as 2.5-volt designs.
Accordingly, what is needed, particularly for low-voltage designs, is a unity gain analog signal amplifier with no DC level shift (so as to preserve dynamic signal range), good frequency response, good current driving capability for capacitive loads and rail-to-rail output voltage swings.